Generally, a photovoltaic device could remain connected permanently, i.e., from the beginning up to the end of its operation, to an AC power grid into which electric energy generated by the photovoltaic device is fed. This, however, would mean that at the time when the photovoltaic device, in case of decreasing insulation, no longer generates a sufficient counter-voltage, a current could flow from the AC power grid into the photovoltaic device. In consequence electric voltage would, for instance also during the night, be present at the photovoltaic device. To avoid absorption of electric energy out of the AC power grid and emergence of electric voltages at the photovoltaic device during the night, it has become a usual practice to disconnect photovoltaic devices from the AC power grid in the evening and to reconnect them again in the morning.
Connecting a photovoltaic device to an AC power grid in the morning shall, on the one hand, happen as early as possible in order to feed electric energy into the AC power grid as early as possible and, thus, to feed as much electric energy as possible into the AC power grid. Similarly, for the same reason, the disconnection from the AC power grid in the evening shall happen as late as possible. On the other hand, connection attempts that have to be discontinued because the photovoltaic device can still not yet provide the required electric power to feed electric energy into the grid should be as rare as possible. Each abortive attempt of connecting a photovoltaic device to an AC power grid means an additional switching cycle for the power switches via which the photovoltaic device is connected to the AC power grid, which are normally air gap switches. This forces to use high-quality air gap switches and to connect them in series with line safety switches, if such line safety switches shall be provided as safety means between the photovoltaic device and the AC power grid, which is the rule. Line safety switches generally have a much lower tolerance with regard to a high number of switching cycles than air gap switches.
In the known methods of and apparatuses for connecting a photovoltaic device to an AC power grid, the connection attempt is aborted and the power switches, via which the grid connection takes place, are reopened in the event of the DC current generated by the photovoltaic device no longer exceeding the minimum connection continuation voltage after connection with the AC power grid. Here, the minimum connection continuation voltage is often selected in such a way that it is the lowest limit for feeding electric energy from the photovoltaic device into the AC power grid.
From DE-T5-11 2007 000 197 a photovoltaic inverter for connecting a photovoltaic device to an AC power grid is known, in which a table comprising seasonal values of minimum connection attempt voltages is stored. When the DC voltage generated by the photovoltaic device reaches the current seasonal value of the minimum connection attempt voltage, a successful connection of the photovoltaic device to the AC power grid via the photovoltaic inverter should be possible. In this way, seasonal influences on the DC voltage of the photovoltaic device, which serves as a criterion for a sufficient performance of the photovoltaic device for a connection attempt, may be taken into account. Particularly, these seasonal influences result from the temperature in the morning, when the connection attempt is undertaken.
From U.S. Pat. No. 7,269,036 B2 a method of and an apparatus for connecting a photovoltaic device to an AC power grid is known, in which the time of connection of the photovoltaic device to the AC power grid in the morning is tuned in that a minimum connection attempt voltage is increased if a previous connection attempt based on the previous value of the minimum connection attempt voltage has failed.
In the two procedures described above, the basic conflict between on the one hand feeding an as high as possible amount of electric energy from the photovoltaic device into the AC power grid and on the other hand making an as low as possible number of abortive connection attempts is not overcome. Either the DC voltage generated by the photovoltaic device from which on a first connection is tried will be set comparatively low to feed as much electric energy from the photovoltaic device into the AC power grid as possible, or it will be set comparatively high to keep the number of finally abortive connection attempts small. Each form of tuning or adaptation of the minimum connection attempt voltage may at best help to better locate the range in which a particular minimum connection attempt voltage has to be set in consideration of both aforementioned criteria.
From JP 08 126207 A a method of connecting a photovoltaic device to an AC power grid is known, wherein a DC current generated by the photovoltaic device is measured, wherein upon reaching a connection voltage by the DC voltage an inverter, which converts the DC voltage into an output AC voltage, is activated and connected to the AC power grid, and wherein the inverter is disconnected from the AC power grid and deactivated again, when the DC voltage falls below a shut-down voltage that is above the peak value of the grid AC voltage.